DE 10 2004 024735 A1 (corresponding to U.S. Pat. No. 7,322,755, which is hereby incorporated by reference in its entirety) describes a camera arrangement having a camera and a prism in which the camera is arranged within a vehicle interior behind a vehicle windshield or other vehicle window (both referred to herein as “windshield” or “pane”), the prism is positioned between the windshield and the camera, the prism is coupled to the windshield by a coupling medium, and the prism has a light entry surface oriented toward the windshield and a light exit surface oriented toward the camera. In particular, the light exit surface of the prism is oriented toward a camera sensor and an objective lens of the camera.
DE 10 2004 024735 A1 describes advantages resulting from the use of prisms with so-called driver assistance cameras. Advantages include robustness of the optical path with respect to environmental effects and miniaturization of the coupling surface (“footprint”) on the windshield required for a given field of view. In order to achieve the robustness and prevent light reflection as much as possible, it is advantageous to press the light entry surface of the prism (e.g., a plane parallel glass plate) directly against the windshield pane with a soft coupling medium. The index of refraction of the optical media is as similar as possible in this case. The abovementioned miniaturization of the footprint is achieved by the prism angle, i.e., the angle between the input and output coupling surfaces. In the special case where the prism angle is 0°, i.e., the input and output coupling surfaces are parallel to one another, footprint reduction does not occur, but it is still possible to achieve an optically robust path. In DE 10 2004 024735 A1, the concept of a prism specifically includes this special case (prism angle=0°).
The curvature of the windshield pane in most types of vehicles and the slanted entrance of the beam when viewing through the normally sharply slanted pane leads to a defocusing effect of the pane when no prism is coupled to the pane. The curvature of the outside of the pane produces a focusing of the light beam with a flat inner side of a pane. This effect is somewhat overcompensated due to the concave surface of the inside of the pane having a somewhat smaller radius of curvature, and the thickness of the pane.
However, when a prism is fitted with a coupling agent to the inside of a curved pane, the defocusing effect of a concave surface fails due to the applied coupling medium and the flat rear side of the prism. This means that the cambered windshield pane and the coupling medium included between the flat prism and the attached pane act together as a lens.
Since the effective radii of curvature of windshield panes are generally different in the horizontal and vertical directions, and furthermore since windshield panes are typically sloped, the lens effect can usually not be completely compensated for by modifying the focus of the camera's objective lens.
DE 10 2008 020 954 A1 describes treating the problem of geometric distortions for a camera arrangement, and describes the light exit surface of a prism being formed as a serrated pattern in cross section, having first and second serrated edges, wherein the first serrated edges run parallel to the flat light entrance surface of the prism. DE 10 2008 020 954 A1 describes the light exit surface being cambered with respect to the serrated structuring in order to produce a lens effect.
DE 10 2009 027 520 A1 (corresponding to U.S. Publication No. 2012/0206601) describes introducing an equalizing sheet that is shaped complementary to the windshield pane into the beam to compensate for distortions. However, geometric distortions compensated for by camera arrangements of the Background Art are not a problem in general. Since the geometric distortions are uniform, they can be eliminated by a computerized image evaluation. Such a computerized image evaluation no longer requires a particularly significant level of effort using the present advanced state of processor technology.
A larger problem, as already described, is that a defocusing lens effect can arise due to the curvature of the windshield pane. The defocusing effect also depends on position and direction. The defocusing is effect is relatively small, but becomes increasingly annoying since driver assistance cameras have high image resolutions. Defocusing causes fuzzy images, and is accompanied by information losses, which should be avoided insofar as possible.
As an overview, when a windshield pane acts as a prism, the pane distorts the external image. When the windshield pane acts as a lens, possibly in conjunction with a coupling path, it defocuses. The defocusing comes about through the curvature of the pane and the different angles of incidence of the light beams, which depend on the direction of viewing. In an example of a windshield pane tilted at 25° with respect to the roadway, horizontally incident light beams arrive at the pane with an angle of incidence of 65°. For a vertical viewing range of ±15°, the incident angle range thus lies between 50° and 80°. A different amount of defocusing occurs for large angles than for small ones, so that this effect cannot be compensated for by changing the placement or direction of the objective lens with respect to the camera sensor.
As described herein, camera arrangements in accordance with embodiments of the present invention relate to this defocusing effect, which is relatively greatly amplified by the coupling medium and the camera prism.